Parts or devices used with miniaturized electronic equipment must be small and thin. On the other hand, as CPU's become highly integrated, they may require a current of several amperes to several tens of amperes. Accordingly, an inductor such as a choke coil used therewith must be small and have a low resistance. That is, the inductor must minimize inductance loss due to superimposed direct current. To make resistance low, a coil conductor should have a large cross sectional area, but this usually requires a large coil. Further, the inductance must have low loss at high frequencies. And since manufacturers are always seeking less expensive parts, inductors should comprise elements of simple shapes that are easy to manufacture. In particular, an inexpensive, miniaturized inductor is needed that can be used with a large current and at high frequencies. However, the high frequency and the large current of a switching frequency make it difficult to both miniaturize the parts that utilize the switching frequency and make the switching circuit highly efficient, because either a switching element increases losses or magnetism of the choke coil is saturated.
Therefore, recently, a circuit system called a multi-phase system has been adopted. For example, in a 4-phase system, four pieces of switching elements and four pieces of choke coils are used in parallel. In this circuit, for example, respective elements are driven at a switching frequency of 500 kHz, DC superimposed of 10 A, and the phase being 90° off. They apparently actuate at the driving frequency of 2 MHz and performance of DC superimposed of 40 A, thereby lowering a ripple current. Thus, the multi-phase system is a power circuit system which can realize large current and high frequency.
The above-mentioned circuit utilizes a coil and a ferrite core of EE type or EI type. The ferrite material, however, has comparatively high permeability and lower saturated flux density in comparison with metallic magnetic materials. Therefore, with a ferrite core, the inductance largely drops due to magnetic saturation, so that the property of DC superimposed tends to be low. Therefore, to improve the property of DC superimposed, the ferrite core is provided with a cavity at one portion, in a magnetic path thereof for use by decreasing the apparent permeability. However, in this method, is difficult to use with a large current because the saturated flux density is low. Having the cavity at one portion in the magnetic path of the ferrite core generates a noisy beating sound in the ferrite core.
In addition, as the core material, it employs Fe—Si—Al or Fe—Ni alloys having a larger saturated flux density than that of the ferrite. But these metallic materials have low electric resistance, so that eddy current loss is large. To compensate, these materials are made thin and laminated with insulating layers, which increases costs.
In contrast, a dust core made by forming metallic magnetic particles has a much larger saturated flux density than that of a soft magnetic ferrite, and has excellent superimposed DC current properties. Therefore, the dust core is advantageous in miniaturizing the circuitry, and no cavity is necessary, which eliminates the beating sound. A core loss of the dust core consists of a hysteresis loss and the eddy current loss, and the eddy current loss increases in proportion to square of the frequency and square of the flowing size of the eddy current. Therefore, the surfaces of the metallic magnetic particles are covered with electric insulation resin for suppressing eddy currents. On the other hand, since the dust core is generally formed at a pressure of more than several ton/cm2, strain increases as a magnetic substance and permeability decreases, so that the hysteresis loss increases. To avoid this, methods to relieve strain are proposed. For example, as disclosed in Japanese Patent Unexamined Publication No. H6-342714, the same No. H8-37107, and No. H9-125108, heat treatments after forming are performed.
To further miniaturize, built-in cores are also proposed, for instance, in Japanese Patent Unexamined Publication No. S54-163354 and the same No. S61-136213. These examples of prior art use cores with ferrite dispersed in resins.
However, when a plurality of inductors are arranged to accommodate multiple multi-phases, installing spaces become large and the circuit becomes expensive. Since a plurality of cores used in the multi-phases have dispersions in inductance values, the ripple current property decreases and the efficiency of the power source also decreases.